The present invention relates to the fabrication of microelectronic devices and/or other micro-scale or nano-scale devices.
Micro-scale devices include semiconductor devices such as microelectronic devices as well as micro-electromechanical and micro-biomechanical devices. For many such micro-scale devices, the sizes and shapes of features therein are determined primarily by photolithographic processing. In such processing, one or more layers of a photoimageable polymer, e.g., a photoresist (a “resist”) or anti-reflective coating (“ARC”), among others, is formed to overlie a substrate. Here, the term “substrate” is used to mean an object underlying the photoimageable layer which is to be patterned in accordance therewith. Thus, the term “substrate” can apply to an object which has a monolithic structure such as a single-crystal semiconductor wafer, ceramic substrate, conductive object or dielectric object. Alternatively, the term “substrate” can also be used to refer to an object which includes a plurality of layers such as a processed wafer or portion thereof such as a chip. All of the layers of such wafer can be collectively referenced as the substrate or just some of the layers can be referenced as the “substrate.”
The photoimageable layer overlying the substrate is selectively exposed according to an image cast thereon by light transmitted through a photomask and focused onto the photoimageable layer. After developing the photoimageable layer, openings exist in the layer which expose selected portions of the substrate while covering other portions of the substrate. Through these openings, subsequent processes such as etching, implanting and/or oxidation, among others, are selectively applied to the exposed portions of the substrate to pattern the features of the micro-scale devices. In this way, the patterns defined by the developed photoimageable layer, e.g., resist layer, are “transferred” to the substrate to reproduce the patterns of the resist layer again in the substrate.
However, this transfer of patterns from resist layer to substrate does not always succeed. When conditions for performing these subsequent processes are particularly harsh, the patterns in the resist layer can become eroded. For example, some anisotropic etching processes (e.g., a “reactive ion etch” or “RIE”) process used to form relatively deep trenches in a substrate can severely erode photoresist and ARC layers, sometimes completely obliterating the photoresist or ARC layer. As a result, the patterns transferred to the substrate may become less sharp, deformed or even missing.
To address this erosion problem of the photoimageable resist and/or ARC layer, an intermediate layer of material having a different composition from the substrate can be disposed as a “hard mask” between the substrate and the resist/ARC layer. The character of the “hard mask” layer is such that it shows little or no erosion during the harsh processing, e.g., a vertical etch process such as RIE which is used to etch the substrate relatively deeply. Typically, a hard mask layer is formed by blanket deposition over the substrate. Thereafter, the photoimageable layer is formed thereon, then exposed and developed by photolithography. The resulting patterns in the photoimageable layer are then transferred to the hard mask layer to form hard mask patterns, by selectively etching portions of the hard mask layer that are exposed by the patterns of the photoimageable layer. Similarly, a subsequent etching process such as RIE is performed to transfer the patterns of the hard mask to the substrate by selectively etching portions of the substrate that are exposed by the patterns of the hard mask layer.
Different materials are available for use as hard mask layers. When the substrate consists essentially of silicon and/or a silicon alloy such as silicon germanium, a hard mask including silicon oxide having various compositions and/or silicon nitride can be used. For particular processes, a hard mask including or consisting essentially of a polycrystalline semiconductor such as “polysilicon” can be used. In particular processes, hard mask layers of a substrate such as include silicon oxide and/or silicon nitride can be etched in accordance with the patterns of an overlying hard mask that consists essentially of polysilicon.
One difficulty posed by the use of polysilicon as a hard mask layer is the ability to properly align a substrate with the polysilicon layer thereon with a photolithographic exposure tool. As deposited normally at a temperature of about 525 degrees Celsius, polysilicon is substantially opaque to light having certain wavelengths including wavelengths of about 5300 angstroms. These wavelengths include light that is typically used in an alignment procedure to form images of alignment features of the substrate and align the exposure tool in accordance therewith. A substantially opaque polysilicon layer disposed on the substrate between the alignment features and the exposure tool greatly increases the difficulty, if not precludes carrying out such alignment procedure.